Airspeed changes to MACH

In the middle of my flight, the SPD on the autopilot changes to MACH speed. If this is a new feature, how do you change it back? If you can’t change it back, how do you calculate MACH speed? Thanks

When you reach cruise, speed will be expressed in Mach. This is not a new feature.

M = u/c
where:

M is the Mach number
u is velocity of the moving aircraft and
c is the speed of sound at the given altitude
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What was your AMSL? At around 28000 ft it changes from IAS in Kts to Mach numbers. When you descend below about 27500 it will change again from Mach to IAS in Kts. On your HUD or docking bar the IAS in Kts should remain.

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@AR_AR answered much better then I did!

Yes crusing altitude changes to mach. The only time it changes is either on descent or at another altitude

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Cruise altitude between Portland and Seattle is 20-22,000 ft and it doesn’t change to mack. I think David is correct in saying itvs when you climb to 28,000 ft that it changes in IF.
I personally don’t know, since I normally don’t ever fly that high.

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I flew that high when I did not know how to land so I took of, go to 30-40k feet and crash using rudder…

Often Fly at FL320 or higher. Tick is working out when to slow down and start to decend. If at FL320 when about 100nm from my destination I slow to about M0.71 (260kts) and start to descend. Once below Fl120 slow to 240kts and below 10000 ft reduce to about 220kts until on approach and slow right down for finals

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The thing to remember is that, even though it seems counterintuitive, in the HUD your airspeed will decrease but your groundspeed will increase the higher you fly. So, for averages I use (NOTE these are merely guidelines I use and not RL) ~300 at 25000 and ~280-290 at 30000 ft… It all depends on your mach speed, which varies for different aircraft. Check wikipedia or other websites for top cruising speed, etc. to give you an estimate of how fast you should be going at certain heights…